In order to decelerate a motor vehicle, the wheels of the motor vehicle are braked. In commercial vehicles in particular, each of the wheel brakes of the wheels comprises brake cylinders, wherein the desired brake pressure in the brake cylinders is generally generated pneumatically.
In a normal braking mode, the brake pressure is adjusted depending on a driver's braking demand determined by the driver of the motor vehicle. Generally, the driver of the motor vehicle transmits his/her driver's braking demand by actuating a brake pedal. In known braking systems, a service-brake valve, which controls the supply of the brake cylinder from a pressure reservoir, is often actuated by means of the brake pedal.
Alternatively to the normal braking mode, in a pressure control mode, the brake pressure is adjusted by a brake control unit at the particular wheel brakes according to the requirements of the brake control unit when corresponding braking requirements have been established. Such braking requirements can be, for example, antilock interventions, when the brake control unit establishes that certain wheels tend to lock. DE 10 2009 058 154 A1 discloses such a braking system which also takes over the adjustment of the brake pressure in the pressure control mode when an external braking demand independent of the driver's braking demand is received, for example the braking demand of a driver assistance system. Driver assistance systems, as systems designed separately from the brake control unit, output signals corresponding to the desired braking power to the brake control unit of the braking system, for example via a data bus.
In the known braking system, the brake control unit carries out a control of the braking systems on the basis of the driver's braking demand and, in addition, on the basis of internal control processes such as antilock interventions or a stability control, and on the basis of the additional external braking demand. The external braking demand is specified to the brake control unit as a setpoint deceleration value, i.e., as a value which represents the deceleration of the motor vehicle desired by the driver assistance system. If external braking demands as well as a driver's braking demand arise in the pressure control mode, i.e., the driver brakes in addition to the external braking demand, the brake control unit adjusts the brake pressure at the particular brakes in accordance with a resultant setpoint deceleration value of the vehicle deceleration. In the known braking system, the driver's braking demand and the external braking demand are additively linked. Alternatively, in the known braking system, in a “maximum” mode, the maximum value is to be formed, by the control unit, from the setpoint deceleration value demanded internally by the braking system due to a driver's braking demand and an externally demanded setpoint deceleration value. An externally demanded braking demand is adjusted only when it is higher than the internal braking demand.
The takeover of the adjustment of the brake pressure in the pressure control mode by a brake control unit when certain wheels tend to lock is known by the designation “antilock braking system” (ABS). The fact is, in every braking operation, only a braking force corresponding to the road friction coefficient can be utilized. If the introduced braking force exceeds the maximum braking force that can be transmitted at one or multiple wheels, the wheels begin to lock, whereby the motor vehicle can become unstable. An ABS system permanently monitors, via measuring signals from speed sensors, the speed of each wheel and, on the basis thereof, ascertains the particular wheel slip. This can take place, for example, by comparing the wheel speed ascertained from the wheel rotational speed with a (computed) vehicle reference speed. If a tendency for the wheel to lock is detected via the wheel slip ascertained in this way, i.e., an ABS slip limit has been reached or exceeded, the brake control unit takes over the control by adjusting the brake pressure. In this case, in a first step, the brake pressure is reduced in order to subsequently regulate the brake pressure of the relevant wheel along the slip limit. In this case, the braking torque is increased again for as long as it takes for a braking torque corresponding to the road friction coefficient to be reached. As a result, in principle, the vehicle is to be nearly optimally decelerated and, simultaneously, the stability and steerability are to be retained.
DE 3829951 A1 discloses a method for carrying out a load-dependent regulation of the brake pressure on a commercial vehicle, which utilizes the components of an existing antilock braking system (ABS) in order to therefore implement an automatic, load-dependent braking function in the normal braking mode that also functions well below the wheel locking limit. In the known method, the brake pressure and, therefore, the braking force distribution are to be controlled below the wheel locking limit in an axle-specific manner, wherein an interaxle brake pressure distribution is automatically controlled—in accordance with the evaluation of the wheel rotational speed signals delivered by the wheel speed sensors—in a slip range below the range in which the ABS function takes effect.
In a method of the generic type for adjusting the brake pressure, for example according to DE 10 2009 058 154 A1, if a need for regulation by the ABS system arises in the pressure control mode after an external braking demand has been received, i.e., it is established that the slip limit has been reached or exceeded at at least one vehicle wheel and, therefore, it is determined that the relevant wheel has a tendency to lock, the antilock function takes over the control by adjusting the brake pressure in the pressure control mode. An implementation of the brake pressure control via ABS during an external braking demand yields disadvantages, in principle. It becomes difficult to adjust the brake pressure in accordance with the demanded vehicle deceleration and, last but not least, the safety of the braking and the driving comfort decrease. In particular, an undesirable jerk occurs, again and again, due to a sudden increase in the brake pressure.